This invention relates to the preparation of expanded layer, smectite clays which provide a regular pore structure for use as a shape selective catalyst, catalyst support, or adsorbent material. More particularly, this invention relates to chromium expanded bentonite clays with the expanded layer pore structure stabilized by thermal treatment in an inert gas atmosphere.
While molecular sieve catalytic materials have been useful in upgrading petroleum stocks, there is a need for new catalytic sieves. The smectite-type clays are capable of layer expansion to form pores with a different shape than the zeolites.
The chemical formulae of all smectite clays are similar to either pyrophyllite or talc clay but substitutions are made for ions in octahedral or tetrahedral sites by ions of lower valency along with addition of an equivalent number of interlayer cations for charge balance (see Deer, Howie, and Zussman, An Introduction to the Rock-Forming Minerals, p. 265 Langman Group Ltd., London (1966)). The smectite clay general formula is then, EQU (Si.sub.8).sup.cn=4 (Al.sub.4).sup.cn=6 O.sub.20 (OH).sub.4
where cn=4 means a coordination number of four and cn=6, a coordination number of six. The substitute four-fold coordination ion typically includes Al.sup.3+ and Fe.sup.3+, and possibly B.sup.3+, or Be.sup.2+. The substitute six-fold coordinated ion usually includes Mg.sup.2+ and possibly Fe.sup.2+, Ni.sup.2+, Co.sup.2+, Li.sup.+, and numerous others known to one of ordinary skill in the art. If the clay is talc, the six-fold coordinated ion is Mg.sup.2+, and the substitutes must be univalent ions such as Li.sup.+. If water is also present between the smectite clay layers, it is weakly bonded to the layers or is present as a hydrate of the interlayer metal cations. Typical smectite clays include bentonite, montmorillonite, beidellite, chlorite, vermiculite, hectorite, and saponite. The bentonite clays used here are primarily montmorillonite clays with silica and other clays as minor contaminates.
We believe it is particularly desirable to prepare expanded smectite clays using chromium-based interlayer complexes since oxides of chromium in catalysts have demonstrated enhanced and/or different catalytic activities. See, for example, Advances in Catalysis, Vol. 30, pp. 1-3, Academic Press, N.Y. (1969); and Vol. 17, p. 225. Expanded layer, smectite clays have been disclosed which are based on various cationic species, including Al, Ti, Fe, Co, Ni, and Zr. For example, see U.S. Pat. Nos. 4,060,480; 4,176,090; and 4,216,188; and also see American Mineralogist 64, 830 (1979), and Clays and Clay Minerals 27, 201 (1979). However, problems have occurred in the attempts to produce expanded clays using chromium complexes. These problems include the inability to produce: (1) an expanded clay which has stable expanded layers in air above 200.degree. C., and (2) an expanded clay with nearly all the available clay layers in the expanded state.
The use of inorganic exchange ions to expand smectite layers is disclosed in U.S. Pat. No. 4,060,480 (Reed and Jaffe). In this patent, the clay is treated with hydroxy-aluminum polymers or oligomers in solution, and the clay is dried and calcined to produce supporting "pillars" between clay layers. These "pillars" maintain the expanded layer state in the clay and leave porosity framed by the "pillars" and the expanded layers. In smectite clays these resulting pores have a rectangular-type opening due to this framing by the "pillars" and clay layers. Thus, the pores have a different shape than the zeolites, which are more circular in shape. In any event, this work by Reed and Jaffe does not suggest either the formation of chromium-based "pillars" between clay layers or the stabilization of the "pillars" by heat treatment in an inert gas atmosphere. A related work on use of aluminum-based polymers to expand the clay layers is Vaughn and Lussier, Preparation of Molecular Sieves Based on Pillared Interlayered Clays (PILC), Fifth International Conference on Zeolites, Naples, Italy, page 94 (1980). In this publication the expanded clays are prepared using aluminum-based oligomers, and the expanded layers are alleged to exhibit thermal stability to about 650.degree. C. While this reference suggests calcination at 500.degree. C. in air produces a stable expanded clay, such a heat treatment results in the collapse of chromium-based clay "pillars" (see Example 4).
Metallic interlayer oligomers of zirconium and titanium have also been utilized to prepare expanded smectite clay materials, and are disclosed in U.S. Pat. No. 4,176,090 (Vaughn et al.). This patent does not, however, disclose chromium oligomers as a possible cation interlayer species, nor is there any suggestion of stabilizing the expanded smectite clay layers by a heat treatment in an inert gas atmosphere.
Organic amine complexes of Fe, Co, and Ni have been used by Loeppert, Mortland, and Pinnavaia to prepare expanded clays as disclosed in Clays and Clay Minerals 27, 201 (1979). Smectite and vermiculite clays are reacted with organic amine complexes of Fe, Co, and Ni to produce expanded clays with (001) planer spacings alleged to range to 18 angstroms for smectite clays and 27 angstroms for vermiculite clays. However, this publication does not disclose chromium expanded clays, and expanded clays prepared with these organic complexes are unstable because calcination would destroy the organic complex, resulting in collapse of the expanded clay layers.
U.S. Pat. No. 4,216,188 (Shabtai) discloses the use of one variety of chromium oligomer to form expanded montmorillonite clay. This patent discloses a chromium oligomer prepared from a mixture of CrK(SO.sub.4).sub.2 and NaOH. Although Shabtai discloses one variety of chromium-based interlayer oligomer for the expanded clays, the starting materials and conditions for the preparation of the oligomer solution and the conditions for reaction of the oligomer with the clay differ from the instant invention.
The Shabtai patent also sets forth apparently equally effective heat treatments to stabilize expanded clay layers: (a) air at 150.degree.-450.degree. C. or (b) inert gas, for example, nitrogen, from 150.degree. C.-450.degree. C. However, the present inventors have established a class of chromium-based expanded clays which require treatment with inert gas only. Unless preceded by the inert gas heat treatment at temperatures of approximately 500.degree. C., air or oxygen heat treatments above 200.degree. C. have a tendency to cause collapse of the expanded layers (see Examples 4, 5, and 10).
Additional work on hydroxy-chromium montmorillonite clays has been done by Brindley and Yamanaka, A Study of Hydroxy-Chromium Montmorillonites and the Form of the Hydroxy-Chromium Polymers, American Mineralogist 64, 830 (1979). Brindley et al. disclose preparation of a hydroxy-chromium oligomer prepared by reacting chromium nitrate with sodium hydroxide. Brindley et al., however, do not disclose any stabilization heat treatment for the chromium expanded clays, and experimental evidence shows differences exist between the process and composition of clay prepared by Brindley versus the instant invention (see Example 5).
It is the general object of the invention to provide a stable porous smectite clay with clay layers expanded by chromium oligomers and a process for preparing these materials. A more specific object of this invention is to provide a bentonite clay expanded by a chromium oligomer and stabilized by an inert gas heat treatment and a process for making these materials. Other objects of the invention will be apparent to persons skilled in the art from the following description and appended claims.